The counties remaining Democratic tend to be all of the following except A.
Border counties

B.
Suburban

C.
Hispanic counties

D.
Heavy concentration of petrochemical industry

B.  Suburban

Explanation:

I'm supposing the question refers to the state of Texas.

Suburban counties in Texas tend to be almost split in half between republicans and democrats, but in some cities like Fort Worth, suburban counties lean more republican, and in others like Austin, they lean more democratic.

Exurban and rural counties are overwhelmingly republican, while urban, industrial, hispanic, and border counties are overwhelmingly democrat.

the answer is b.

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